Dissipative temporal Kerr solitons in optical microresonators enable to convert a continuous wave laser into a train of femtosecond pulses. Of particular interest are single soliton states, whose sech2 spectral envelope provides a spectrally smooth and low noise optical frequency comb, and that recently have been generated in crystalline, silica, and silicon-nitride resonators. They constitute sources that are unique in their ability to provide short femtosecond pulses at microwave repetition rates. Likewise, they provide essential elements to realize chip-scale, integrated frequency combs for time-keeping, spectroscopy, navigation or telecommunications.
However, to date, the dynamics of this class of solitons in microresonators remains largely unexplored, and the reliable generation of single soliton states remains challenging.
Microresonator frequency combs (Kerr combs) have opened a novel research area at the interface of micro- and nano-photonics and frequency metrology. Kerr combs are generated in high-Q millimeter- or micronscale resonators via parametric processes driven by continuous wave (CW) laser. Kerr combs have attracted significant attention over past years due to unprecedented compactness, demonstrated octave-spanning operation, repetition rates in the microwave domain (>10 GHz), and the ability to be operated in low noise regimes. They promise chip-scale optical frequency combs connecting RF to optical domain that could make metrology ubiquitous, widely accessible beyond specialized metrology laboratories.
Recently, it has been demonstrated that Kerr combs can be operated in the regime of temporal dissipative Kerr solitons (DKS). DKS allow for fully coherent optical frequency combs (soliton combs) that can be sufficiently broadband for self-referencing via soliton induced Cherenkov radiation, and provide access to stable ultrashort pulses of tunable duration at microwave repetition rates. As mentioned, of particular interest are single soliton states that exhibit a spectrally smooth sech2 envelope. Such soliton based frequency comb sources have a wide range of applications including molecular spectroscopy, coherent data transmission, arbitrary waveform generation, optical clocks or astrophysics, and more generally in applications where short pulse duration at microwave repetition rate is desirable.
Originally discovered to spontaneously form in crystalline MgF2 resonators (and for the first time externally induced in optical fiber cavities), DKS have been demonstrated in a variety of high-Q resonator platforms, ranging from silica wedge resonators, to Si3N4 photonic chips and compact crystalline resonators pumped via distributed feedback lasers. Due to the recent nature of these findings, the soliton formation process and its dynamics remain to date largely unexplored.
While solitons have been reported in a number of platforms, the soliton generation procedures in high-Q microresonators are inherently stochastic (techniques used in optical fiber cavities are technically impractical due to much shorter round-trip time of microresonators). While CW laser tuning and “power kicking” schemes were proposed for soliton generation, these techniques presently do not allow to control the number of solitons formed in the resonator.
Another important question is the possibility of deterministic manipulation of states with multiple solitons in microresonators. Even though the states with various number of solitons could be generated in optical microresonators, the transitions between them take place stochastically via pairwise interactions of solitons when the pump is tuned, and cannot be predicted so far. Due to these effects, deterministic generation of the single soliton state still represents an outstanding challenge.
One more challenge is the non-destructive monitoring of the soliton state. The soliton regime in microresonators is fragile (though self-sustainable) and is not persistent against significant thermal drifts and other external perturbations. The reported passive lifetime of DKS achieves several hours in a stable laboratory environment, however, no technique is known to enable feedback stabilized control of soliton state, preventing it from decay.